For me, D failed to replace C++ because of lack of design. It is more a mix of great features. But once you start learning the details simple things can get very complicated.
For example, function arguments can be "in", "out", "inout", "ref", "scope", "return ref" - and combinations.
Another example is conditional compilation. Great when used sparely, but can otherwise make it very difficult to understand how the code flows.
In the end, reading the source code of the standard library convinced me against it.
(The source code for the C++ standard library is much worse, of course).
For example, function arguments can be "in", "out", "inout", "ref", "scope", "return ref" - and combinations.
Another example is conditional compilation. Great when used sparely, but can otherwise make it very difficult to understand how the code flows.
In the end, reading the source code of the standard library convinced me against it.
(The source code for the C++ standard library is much worse, of course).